Ac-dc transfer standards provide an essential link for the determination of ac electric quantities in terms of the SI units. Thermal converters are by far the most widely used form of primary ac-dc transfer standards. The basic principles of ac-dc transfer measurements are presented together with background to the performance and basic limitations of thermal converters as transfer standards. Developments in basic converters, voltage transfer standards and current transfer standards are reviewed, with emphasis on developments since 1985. Current activities and possible future directions are discussed.